The present invention relates to a hydraulic brake system with slip control, in particular for automotive vehicles, comprising a master cylinder pressurizable by a hydraulic power booster, in which system valve means are inserted between the master cylinder and the wheel brakes connected to the master cylinder which allow to remove pressure fluid from the wheel brakes, wherein the pressure fluid taken from the wheel brakes can be replenished out of the pressure chamber of the hydraulic power booster, and wherein a stroke limitation of the brake pedal is effected during slip control.
A hydraulic brake system incorporating the preceding features is known from German printed and published patent application No. 3040562. This known brake system comprises a hydraulic power brake booster is which a booster pressure proportional to the actuating force is determinable by a brake valve actuatable by a pedal. The hydraulic power booster serves to actuate a tandem master cylinder, to whose working chambers wheel brakes are connected. The connections between the working chambers of the tandem master cylinder and the wheel brakes connected thereto can be interrupted by electromagnetically actuatable valve means. In addition, further valve means are allocated to the wheel brakes which serve to remove pressure fluid from the wheel brakes in case of need, so that the effective braking pressure will drop, as a usual result whereof the vehicle wheel concerned will be re-accelerated sufficiently in the event of an imminent locked condition.
Pressure fluid that has been removed from the wheel brakes during brake slip control will be replenished out of the pressure chamber of the hydraulic power booster, and the pressure fluid prevailing in the pressure chamber of the hydraulic power booster is fed into the corresponding working chambers via additional valve means through the sleeves of the master cylinder pistons. The known brake system necessitates a stroke limitation with a view to ensuring the availability of a minimum volume in the tandem master cylinder. For this purpose, upon pressurization of the working chambers of the tandem master cylinder, a so-termed positioning sleeve will be simultaneously pressurized in the brake's release position in opposition to a spring force, as a result whereof the positioning sleeve will move to bear against a stop of the booster piston and prevent further depression of the booster piston and of the master cylinder pistons owing to the ratios of effective surfaces prevailing.
As a function of the ratios of friction values prevailing, in the brake unit described the brake pedal may be reset through the booster piston in opposition to the actuating force exerted on the brake pedal. On the other hand, it is possible in the event of low friction values, in the presence of which the limit of wheel lock will be attained at relatively low master cylinder pressures already, that upon commencement of the slip control the brake pedal will perform an additional travel in the actuating direction, prior to the stop of the booster piston having moved into abutment on the positioning sleeve. A like mode of function may erroneously lead the vehicle driver to think that the entire brake system is defective. As a whole, the known brake system is of relatively complicated design which is in first line condition by the fact that the booster piston is partially guided in the positioning sleeve, while the positioning sleeve itself needs to be arranged slidably in the housing in a fashion pressure-sealed at its outer periphery.
It is therefore an object of the present invention to structurally simplify the hydraulic brake system of the species initially referred to and to prevent a depression of the brake pedal through its full travel during the control at low coefficients of friction of the road surface.